In the recent years, the necessity of increasing the steelmaking process efficiency and productivity has become more urgent, due to rising production costs and also due to the restrictions imposed upon steel plants by environmental regulations.
One of the successful routes for steelmaking, which is being increasingly promoted, is the gas based Direct Reduction of Iron Ore to produce Direct Reduced Iron (DRI), also known in the steel industry as sponge iron, by circulating a reducing gas through a moving bed of particulate iron ore at temperature of the order of 700° C. to 1100° C. Oxygen is removed from the iron ore by chemical reduction for the production of highly metallized DRI.
Some of the advantages of direct reduction plants are the wide range of production capacity, that the metallic iron is produced in solid form with low sulfur and silicon content, and that the resulting DRI may be used as raw material for the electric furnace and may constitute the whole charge thereof.
Additionally, and as a peculiar advantage of the technology proposed, given that a part of the CO2 produced as by product of the reduction reactions is selectively removed from the process, total CO2 emitted in the atmosphere may be considerably reduced if compared with others routes for steel production.
The reducing agents utilized in the direct reduction plants are hydrogen and carbon monoxide, obtained by reformation of natural gas in an external catalytic reformer or “in situ” within the iron reduction system. Nevertheless, a direct reduction plant can be also designed for utilizing other sources of energy available in the form of gases from coke ovens, blast furnaces, coal or oil gasification, natural gas, exhaust gases containing hydrogen and carbon monoxide arriving from other chemical/metallurgical processes, etc.
A possible source of reducing gas is the excess gas produced in the combination of a plant for the production of pig iron based on the use of coal (for example a blast furnace or a plant known in the industry with the tradename Corex) and a direct reduction reactor. Corex plants or blast furnaces produce pig iron using gasified coal by partial combustion with an oxygen-containing gas. The exhausted reducing gas withdrawn from this process, still containing H2 and CO, can be utilized for reduction, after removal of at least a portion of H2O and CO2.
U.S. Pat. No. 5,238,487 to Hauk et al. discloses a process comprising a melter-gasifier, a first reduction reactor and a second reduction reactor wherein DRI is produced using directly reducing gas effluent from said first reactor. As indicated in this patent, the effluent reducing gas, after being only cleaned, is mixed with dewatered spent reducing gas and treated in a CO2 removal unit. The gas leaving the decarbonation station is then heated in a heat exchanger and finally subjected to a partial combustion to reach the right temperature required for the reduction reaction. Additionally, this patent teaches to use sulfur oxides and chlorine to inhibit carbon monoxide decomposition. All embodiments of this patent however utilize heat exchangers that consume a fuel for heating the reducing gas prior to the partial combustion heating stage.
U.S. Pat. No. 5,676,732 to Viramontes-Brown et al. discloses an improved method and an apparatus for utilizing in a direct reduction plant the excess exhausted gas from a first reduction reactor, which receives reducing gas from a melter-gasifier. Said method suggests to use a catalytic reactor, or shifter, for adjusting the composition of the gas stream effluent from said first reactor in order to avoid carbon deposition and corrosion in the gas heater required to heat fresh gas before feeding it into the reduction reactor. In order to get the maximum yield of H2 product from the CO shift conversion, a special catalyst in a fixed bed reactor is used. For this reason, Syngas has to be further treated in order to remove substances that are poisonous for the catalyst.
Referring now to Syngas from a gasifier as alternative source of reducing gas, U.S. Pat. No. 6,149,859; and U.S. Pat. No. 6,033,456 to Jahnke et al. describe an integrated process for supplying high-pressure Syngas from a gasifier to a direct reduction plant. As in the prior art, this patent suggests to treat the Syngas in a shifter with the purpose of changing its composition in order to avoid carbon deposition when said gas is heated at a temperature higher than 400° C. (condition commonly achieved in a typical process gas heater of a Direct Reduction Plant). In this way, the conditioned gas stream, after being treated in a dedicated unit to remove CO2 and being expanded to the pressure of the direct reduction circuit, is ready for being used as make up in the DRI process.
WO-A-2008/146112 discloses the additional possibility of having, in a process as described in U.S. Pat. No. 6,149,859 and U.S. Pat. No. 6,033,456, a single absorption unit wherein the acid-gas content is removed from a combined stream of both the Syngas produced in the gasifier and the recycle reducing gas from the reduction reactor.
U.S. Pat. No. 5,846,268 to Diehl et al. discloses a process for producing liquid pig iron or liquid steel pre-products and DRI from iron ore. The process shown in this patent is much similar to the process described by U.S. Pat. No. 5,238,487 to Hauk et al. where a reducing gas, derived from the gasification of coal is used for reducing iron ore in a first reduction shaft furnace and the exhausted reducing gas effluent from said first shaft furnace is utilized for producing more DRI in a second shaft furnace. This patent teaches several ways of using heat of the gas stream effluent from the second shaft furnace for preheating a portion of the same gas stream which is then utilized as fuel in a fired gas heater, but does not teach or suggests using said heat to preheat the stream of reducing gas fed to the reduction reactor.
None of the above patents teach or suggest the distinctive features of the present invention which overcome a number of disadvantages of the prior art and provide a more efficient method and apparatus for producing DRI utilizing gas derived from coal gasification in a gasifier or derived from a melter-gasifier, for example, using heat from the top gas effluent from the reduction reactor for heating the reducing gas to be fed to said reactor without consuming any additional fuel and within the practical limits of the degree of oxidation of the reducing gas for an efficient reduction of iron ore.
An additional advantage of the present invention is that the carbon dioxide emissions to the atmosphere can be decreased because there is no combustion in the heat exchanger for raising the temperature of the reducing gas prior to second heating stage of partial oxidation with oxygen.